What was found
AIRCRAFT LEVEL AT 12,000 FEET. 325 KNOTS FOR TEN MINTUES WITH AP NAV AND ALT ANNUNCIATION. ALT PRE-SELECT AT 12,000 FEET. PILOT, LT SEAT MADE COMMENT ABOUT A FEELING. RT SEAT PILOT THEN NOTICED THE ACFT PASSING THROUGH 12,250 FEET JUST WHEN ALT ALERT HAD SOUNDED. ACFT WAS CLIMBING FOR COMMAND BARS WHICH WERE AT 7/8 DEGREES. CREW PUNCHED OFF AUTOPILOT AND SLOWED CLIMB TO A STOP AT 12,650 FEET. ATC ADVISED OF PROBLEM AND DEVIATION. CREW MANUAL FLEW ACFT BACK TO 12,000 FEET. A LANDING AT DESTINATION MADE WITHOUT FURTHER INCIDENT. NO FLAGS NOTED DURING OCCURRENCE. GROUND FAULT TEST SHOWED NO FAULTS OCCURRED. SUSPECT NR 1 ADC. IT WAS REMOVED AND REPLACED. BOTH ALT'S TESTED. ACFT RETURNED TO SERVICE.